Arduino: Open source hardware

Check out Arduino, a totally cool tiny embedded computer suitable for creating smart things - art, robotics, sensors and controllers. You program it on your laptop, and download the program to the device. I tried it, it was pretty easy. Had to touch wires and stuff though :)

It appears to be a vibrant community with lots of people building compatible boards as well as add-ons which they seem to call "Shields". For example here's a shield to allow wireless communication between two Arduinos.

If you want to learn more, check out books called "Physical Computing" and "Making things Talk".